The present invention relates to wide format printers and, in particular, it concerns a feed mechanism for wide format flexible substrates.
It is known to provide wide format printers for printing on wide format flexible substrates such as paper, card, vinyl and other polymers, and textiles. As illustrated in FIG. 1, feed mechanisms for flexible substrates typically employ an arrangement of rollers in which the flexible substrate is driven by friction contact with a traction roller 10. Contact pressure is maintained by a pressure roller 12 which is pressed against the traction roller by a clamping system. The clamping system is generally implemented as pneumatic pistons 14 acting on the axle of pressure roller 12 at its ends.
While very effective for small formats, this type of feed mechanism becomes problematic with wider formats, particularly with widths in excess of about three meters. At these dimensions, slight bending of the rollers leads to significant variations in contact pressure between the rollers from a maximum value at the ends down to a minimum, or even zero, at a central portion. As a result, traction at the central portion of the rollers is reduced or altogether lost, leading to wrinkling of the substrate.
Various approaches have been proposed for improving traction along the rollers. A first approach employs the use of a pressure roller with a diameter which varies along its length so that its central portion bulges slightly outwards. This provides a partial solution, somewhat offsetting the variations in contact pressure along the length of the rollers. This offset, however, is only effective for specific clamping forces, any variation in clamping forces leading again to uneven contact pressure and consequent wrinkling of the substrate.
A second approach, illustrated in FIG. 2, replaces the pressure roller with a number of small pressure wheels 16, each with its own pneumatic piston 14, spaced along the length of traction roller 10. Although this approach ensures sufficient traction for effective feed of the substrate, the narrow pressure wheels generally form visible depressions in the substrate, thereby degrading the output quality.
There is therefore a need for a feed mechanism for wide format flexible substrates which provides effective traction across the width of a traction roller without forming visible depressions in the substrate.